A non-drone player was necessary, as Azad would never have acknowledged defeat by a drone. But, defeat by a human accomplished the Culture's goal and Gurgeh was the best bet (or at least the best available one) on the desired outcome.

I see their selection as mostly being about Gurgeh having enough pride to accept the small cheat, coupled with enough skill to not actually need the AI assistance. It's been a while since I read TPoG last, but my recollection is that there were a handful of players at essentially the same level as Gurgeh and from a pure skill and intuition level, I suspect any of them would have worked, but only Gurgeh fell for the entrapment.